Until recently all industrial robots installed in factories were separated from people by safety barriers and interlocks to prevent the robots from colliding with a human and possibly causing injury. Large robots can carry payloads of hundreds of kilograms and are therefore potentially very dangerous due to the masses involved. However, even small assembly robots, which often operate at high tool speeds of 7-10 meters per second, have enough energy to seriously injure humans though their payloads are typically 3 kg or less.
Recently there has been growing interest in allowing robots to work safely next to people in the same working volume without the need for expensive and cumbersome safety screens. This class of robots is referred to in the industry as “Collaborative Robots.”
In 2016, an ISO standard ISO_TS 15066 2016 “Robots and Robotic Devices—Collaborative Robots,” was published listing safe levels for collision forces that would not cause injury to people. Collision force is determined by the robot's and the payload's kinetic energy, the stopping distance and commanded motor torque. The stopping distance is determined by the kinetic energy, the compliance (stiffness) of the part of the human body that is struck, the compliance of the robot structure that strikes the person, and whether the collision occurs in free space or traps a part of the body against a rigid surface. The safe stopping distance can typically vary from a few millimeters to a few centimeters, depending on these factors. In order to comply with this standard, many robots users have had to dramatically slow down their operating speed if there was any chance the robot could collide with a human.
Various efforts have been made to reduce collision forces by means of sensors. For example, laser zone sensors have been used to detect if a person is entering a specified zone and to subsequently slow down or stop a robot. However, these are quite expensive, typically costing thousands of dollars, and for small robots are difficult to justify when compared to a safety barrier. Also, even if a collision does not occur, this approach negatively impacts the robot's cycle time and productivity whenever a person works close to the robot, which is the real goal and benefit of collaborative robots.
Touch and force or torque sensors can be added to the robot, but these typically only affect the tool tip or part of the robot, and require a collision before the control system can react and attempt to slow the robot. Consequently, to allow for an adequate reaction time, the robot must be operated at a slower, less productive speed virtually all of the time. These sensors also add cost, and if depended on for safety must be made redundant or fail-safe.
Work is ongoing in evaluating capacitive distance sensors in an attempt to sense a human electrical field from a great enough distance to slow down a robot. This work has not been proven safe as clothing and metal objects can interfere with this type of sensing. Also, like laser zone sensors, the robot and work cell cycle time are adversely affected even if a human is in the vicinity of the robot, but no collision occurs.